:::demo 示例

```html
<template>
    <div>
        <button @click="checkAll()">全选切换</button><br/><br/>
        <v-checkbox-group v-model="cities4">

            <v-checkbox :label="item.value" v-for="(item,index) in checkedCities4" :key="index">{{item.name}}</v-checkbox>

        </v-checkbox-group>
    </div>
</template>

<script>
    export default{
        data() {
            return {
                checkedCities4:[
                    {name:'上海',value:1},
                    {name:'北京',value:2},
                    {name:'深圳',value:3},
                ],
                cities4:[2]
            };
        },
        methods:{
            checkAll(){
                if (this.cities4.length === this.checkedCities4.length){
                    this.cities4 = [];
                }else{
                    this.cities4 = this.checkedCities4.map(x=>x.value);
                }
            }
        }
    }
</script>
```
:::